Thermally integrated hotbox combining a steam reformer with SOFC stacks

ABSTRACT

A thermally integrated hotbox apparatus combining a steam reformer, a plurality of solid oxide fuel cell (SOFC) stacks, a plurality of oxidant manifolds, and at least one heat extractor. The steam reformer occupies a central position in the hotbox, around which are disposed in spaced-apart relation a plurality of SOFC stacks. A burner may be associated with the steam reformer, either within or outside the hotbox. An oxidant manifold is disposed between each pair of adjacent SOFC stacks. A heat exchanger is incorporated between an SOFC stack and an oxygen manifold. The hotbox design optimally captures thermal heat from the SOFC stacks for use in producing steam and operating the endothermic steam reformer. The apparatus reduces duty cycle of the burner, which produces heat and steam needed for operation of the endothermic steam reformer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.16/672,663, filed Nov. 4, 2019, now allowed, which claims benefit ofU.S. provisional application No. 62/760,964, filed Nov. 14, 2018; theaforementioned applications in their entirety incorporated herein byreference.

FIELD OF THE INVENTION

This invention pertains to a thermally integrated hotbox apparatus thatcombines a steam reformer with a group of solid oxide fuel cell stacks.

BACKGROUND OF THE INVENTION

A solid oxide fuel cell (SOFC) stack operating at high temperatures ofabout 550° C. to 850° C. provides an overall fuel to electric efficiencyof about 40 to 50 percent, which results in a significant production ofexcess or waste heat. As known in the art, the term “SOFC stack” refersto a structure comprising a plurality of individual solid oxide fuelcell repeat units electrically connected in series. Each individualsolid oxide fuel cell repeat unit comprises an oxygen electrode(cathode) wherein oxygen is reduced with a flow of electrons to oxideions; a solid oxide electrolyte that transports oxide ions so producedat the cathode to a fuel electrode; the fuel electrode (anode) whereinthe oxide ions and a fuel, such as hydrogen, carbon monoxide or amixture thereof, are contacted to produce, respectively, water, carbondioxide, or a mixture thereof with concomitant production of electrons;and an external electrical circuit that collects the electrons soproduced and delivers them to the cathode while also being available forperforming useful work. In SOFC systems at least one stack or more aredisposed within a structural housing referred to as a “stack hotbox”.Recuperating the waste or excess heat from the environs around thestack(s) but defined within the stack hotbox would enable higher fuel toelectric efficiency.

The SOFC stack operates on a fuel source comprising hydrogen, carbonmonoxide, or a mixture thereof. Hydrogen and carbon monoxide can besupplied to the stack via a steam reformer (SR) wherein a hydrocarbonfuel, such as natural gas or methane or diesel, is contacted with steamand converted in an endothermic process into a synthesis gas (syngas)comprising a mixture of hydrogen and carbon monoxide and lesserquantities of carbon dioxide and water. The heat required for theendothermic steam reforming is typically generated in an associatedburner module, wherein a portion of the hydrocarbon fuel is combustedthereby generating the heat needed for the steam reformer.

Advantages would be achieved if the waste heat radiating from any SOFCstack could be removed from the environs of the stack and utilized in aproductive manner. Actively removing the waste heat from the stack wouldlower the temperature of the stack hotbox, which in turn wouldbeneficially result in slower degradation and improved durability of theindividual solid oxide fuel cell repeat units. Likewise, a lowertemperature of the stack hotbox would advantageously lower requirementsfor cathode air flow into the SOFC due to reduced cooling needs, whichhas the advantage of reducing pressure on seals, pumping loads, andsystem parasitics. Moreover, it would be advantageous to achieve a fuelutilization of greater than about 80 percent in the stack withcomplementary removal of increased heat. Any improvement in stack fuelutilization, however, is tied to improved thermal management of thesteam reformer. Specifically, it would be advantageous to recover wasteheat from the SOFC stack for use in generating the steam and heat neededto satisfy the heat requirements of the steam reformer.

The skilled person will appreciate that the thermal integration of thesteam reformer with the SOFC stack(s) is challenging. Both the steamreformer and the SOFC stack(s) have their own array of inlet and outletmanifolds. Moreover, the integrated hotbox would require connectingappropriately designed manifolds between the stream reformer and theSOFC stack(s). Typically, the inlet and outlet manifolds are constructedof metallic parts; while each SOFC stack is constructed of layers ofceramics. Seals and contacts between metallic and ceramic parts can beproblematical, due to differences in thermal expansion and heattransfer, thereby leading to stack failure under operating conditions.In a well-integrated hotbox, the number of connecting manifolds shouldbe minimized. Moreover, seals and contacts between metals and ceramicsshould be minimized so as increase construction durability and heatintegration.

SUMMARY OF THE INVENTION

The invention described herein provides for a novel hotbox apparatusintegrating a steam reformer with a plurality of solid oxide fuel cellstacks, so as to provide for improved thermal integration. Thus, thethermally integrated hotbox apparatus of this invention comprises:

-   -   (a) a steam reformer comprising (i) a fuel inlet and a steam        inlet, (ii) a reforming zone disposed in fluid communication        with the fuel and steam inlets; and (iii) a reformer outlet        disposed in fluid communication with the reforming zone;    -   (b) a plurality of solid oxide fuel cell stacks disposed around        the steam reformer, further disposed in spaced apart relation to        each other and to the steam reformer;    -   (c) wherein each solid oxide fuel cell stack comprises a stack        fuel inlet disposed in fluid communication with the reformer        outlet, and further comprises a fuel exhaust outlet; further        wherein both the stack fuel inlet and the fuel exhaust outlet        are disposed in fluid communication with a fuel side of each of        the plurality of solid oxide fuel cell stacks;    -   (d) a plurality of oxidant manifolds disposed around the steam        reformer, such that each oxidant manifold is disposed in between        a pair of adjacent fuel cell stacks;    -   (e) wherein the plurality of oxidant manifolds alternatingly        comprise an oxidant inlet but no oxidant outlet; and wherein a        remaining plurality of oxidant manifolds having no oxidant inlet        comprise an oxidant outlet; and further wherein each oxidant        manifold defines an interior plenum fluidly communicating with        an oxidant side of each of the plurality of fuel cell stacks;        and    -   (f) at least one heat extractor disposed in between one of the        solid oxide fuel cell stacks and its adjacent oxidant manifold;        the at least one heat extractor having a water inlet and a steam        outlet, wherein the steam outlet is disposed in fluid        communication with the steam inlet to the steam reformer.

The hotbox apparatus of this invention thermally integrates a pluralityof solid oxide fuel cell stacks with a steam reformer, and with aplurality of oxidant manifolds, and with at least one heat extractor.The hotbox apparatus of this invention “taken as a whole” offersimproved efficiency of function. Specifically, the hotbox apparatusdescribed herein streamlines design of the fuel and oxidant manifolds,their inlets, and their outlets to a small number of repeatable units.The design permits optimal capture of excess stack heat and results in auniform temperature distribution around the stack, while allowing stackseals to maintain an advantageously reduced temperature. Moreover, eachstack desirably then maintains a similar electrical power ensuringimproved balanced load distribution and improved lifetime for the entireSOFC system. The design advantageously reduces contact areas betweenceramic and metallic parts. Additionally, the design reduces or avoidsthe duty cycle under steady state operating conditions of any burnerdisposed internal or external to the hotbox, which might be associatedwith the steam reformer. The design of this invention results in a morecompact hotbox having a greater ease of manufacture, a higherefficiency, and an improved longevity through overall hotboxconstruction durability.

DRAWINGS

FIG. 1 depicts a top and closed view of an embodiment of the thermallyintegrated hotbox apparatus of this invention.

FIG. 2 depicts a top and pulled-apart view of the embodiment of theinvention illustrated in FIG. 1 .

FIG. 3 illustrates in isometric and pulled-apart view an embodiment ofthe thermally integrated hotbox apparatus of this invention.

FIG. 4 illustrates in isometric view an embodiment of thefully-assembled thermally integrated hotbox apparatus depicted in FIG. 3.

FIG. 5 illustrates in transverse horizontal view one embodiment of amesh substrate employable in a stream reformer suitable for theapparatus of this invention.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, this invention provides for a novel thermallyintegrated hotbox defined by a hotbox housing comprising the followingcomponents:

-   -   (a) a steam reformer disposed at a center of the hotbox, the        steam reformer comprising (i) a fuel inlet and a steam        inlet, (ii) a reforming zone disposed in fluid communication        with the fuel and steam inlets; (iii) a reformer outlet disposed        in fluid communication with the reforming zone;    -   (b) a burner disposed in thermal communication with the steam        reformer;    -   (c) a plurality of solid oxide fuel cell stacks disposed around        the steam reformer, in spaced apart relation to each other and        to the steam reformer;    -   (d) wherein each solid oxide fuel cell stack comprises a stack        fuel inlet disposed in fluid communication with the reformer        outlet, and further comprises a fuel exhaust outlet; further        wherein both the stack fuel inlet and the fuel exhaust outlet        fluidly communicate with a fuel side of each of the plurality of        solid oxide fuel cell stacks;    -   (e) a plurality of oxidant manifolds disposed around the steam        reformer, such that each oxidant manifold is disposed in between        a pair of adjacent fuel cell stacks;    -   (e) wherein the plurality of oxidant manifolds alternatingly        comprise an oxidant inlet but no oxidant outlet; and wherein a        remaining plurality of oxidant manifolds with no oxidant inlet        comprise an oxidant outlet; and further wherein each oxidant        manifold defines an interior plenum fluidly communicating with        an oxidant side of each of the plurality of fuel cell stacks;        and    -   (f) at least one heat extractor disposed in between one of the        solid oxide fuel cell stacks and its adjacent oxidant manifold;        the at least one heat extractor having a water inlet and a steam        outlet, wherein the steam outlet is disposed in fluid        communication with the steam inlet to the steam reformer.

In one embodiment of this invention, connections between each fuel cellstack and each adjacent oxidant manifold are made solely at the edges ofthe stack and the edges of the oxidant manifold. No connections are madeon adjacent faces of the stack and the oxidant manifold.

In another embodiment of this invention, a steam reforming catalyst isdisposed within the reforming zone, the steam reforming catalystcomprising a porous substrate having an ultra-short-channel-length andhaving a Group VIII metal deposited thereon.

FIG. 1 illustrates, from a top closed view, an embodiment of thethermally integrated hotbox (10) of this invention. Within a hotboxhousing (19) are disposed four SOFC stacks (1), located on opposite endsof two perpendicular axes (e.g., x, y), spaced apart around acentrally-located steam reformer (3) comprising a fuel inlet (5).Between each pair of adjacent SOFC stacks (1) is disposed an oxidantmanifold (11). Four such manifolds are seen. FIG. 1 illustrates that thefour fuel cells (1) and the four oxidant manifolds (11) are seamed attheir edges (17, bold lines) to form a solid body.

FIG. 2 depicts the embodiment of the thermally integrated hotbox (10) ofFIG. 1 , as illustrated from a top, pulled-apart view. The hotboxhousing (19) containing the steam reformer (3), four solid oxide fuelcell stacks (1), and four oxidant manifolds (11) are disposed again inthe manner shown in FIG. 1 . A reformer outlet (2) from the steamreformer (3) fluidly connects the stream reformer (3) to each SOFC stack(1); such that reformate is fed as a stack fuel to the stacks (1),specifically to the anode (fuel electrode) side of each stack. Each fuelcell stack (1) includes an anode waste (fuel exhaust) outlet (9) exitingfrom the anode side of the stack. In this view, solid arrows (13, 15) atthe oxygen manifolds (11) indicate the flow of oxidant into and out inalternating order, such that inlets (13) provide for a flow of oxidantinto the manifolds (11) and outlets (15) provide for a flow of wasteoxidant from the alternating manifolds (11). The dashed arrows indicatethe oxidant flows, from one oxidant manifold (11) through the cathode(oxidant electrode) side of the adjacent SOFC stacks (1) and thenpassing through and out of the next-neighboring oxidant manifolds (11).At least one heat extractor (7) is positioned in between a fuel cellstack (1) and an oxidant manifold (11), the heat extractor (7) fluidlyconnected to the steam inlet of the steam reformer (3).

FIG. 3 depicts the embodiment of FIG. 1 in isometric, pulled apart viewand absent the housing. Again, four solid oxide fuel cell stacks (1) aredisposed on opposite ends of two perpendicular axes (e.g., x, y), spacedapart around a centrally-located steam reformer (3). A reformer fuelinlet (5) feeds a hydrocarbon fuel into the steam reformer (3). Areformer outlet (not visible) fluidly connects the steam reformer (3) toeach fuel cell stack (1), particularly, to the fuel electrode (anode)side of each fuel cell stack (1). Each fuel cell stack (1) includes afuel exhaust outlet (9). Between each solid oxide fuel cell stack (1) isdisposed an oxidant manifold (11), one between each adjacent pair offuel cell stacks (1). Alternatingly, the oxidant manifolds (11) includean oxidant inlet (13) or an oxidant outlet (15). At least one heatextractor (7) is positioned between a fuel cell stack (1) and an oxidantmanifold (11) with a steam outlet (not shown) connecting to the steaminlet to the reformer (3).

With reference to FIG. 4 , there is depicted an isometric exterior viewof a fully assembled hotbox unit (10) absent the housing, but as seencontaining four solid oxide fuel cell stacks (1) disposed around acentrally-located steam reformer having a fuel inlet (5). Each fuel cellstack (1) includes a fuel exhaust outlet (9). Between each adjacent pairof solid oxide fuel cell stacks (1) is disposed an oxidant manifold(11). The oxidant manifolds (11) include in alternating order an oxidantinlet (13) and oxidant outlet (15). The fully assembled hotbox (10) isprovided with seals and connections (17) only along seams wherein edgesof each fuel cell stack (1) contacts edges of adjacent oxidant manifolds(11).

FIG. 5 depicts in horizontal transverse view an embodiment of a reformer(3) and an associated burner (21) that are suitably employed in theintegrated hotbox of this invention. In this embodiment, the burner (21)occupies a cylindrical core around which an annular-shaped steamreformer (3) is disposed. The reformer (3) comprises a mesh substrate(23), preferably, of ultra-short-channel-length as detailed hereinafter,onto which is supported (for example, coated) particles of a reformingcatalyst (25).

The thermally integrated hotbox of this invention comprises a hotboxhousing constructed in any size and shape convenient to house allcomponents including a steam reformer, a plurality of SOFC stacks, aplurality of oxidant manifolds, at least one heat exchanger, optionallya burner, and then various inlets, outlets, and conduits associated withthe aforementioned components. Materials of construction of the hotboxhousing, the inlets, outlets, and conduits associated therewith shouldbe capable of withstanding temperatures to which each component isexposed, such temperature usually ranging from greater than about 600°C. to less than about 1,000° C. Suitable non-limiting materials ofconstruction include, for example, nickel-chromium alloys andnickel-chromium-iron alloys, such as INCONEL® brand, HASTELLOY® brand,and HAYNES® brand of alloys. The hotbox housing is typically lined oninterior and/or exterior surfaces with a thermal insulator, as is knownto the skilled person; the insulator substantially retaining heat withinthe hotbox without undue thermal losses to the external environs.

In the unique design of this invention, the steam reformer is disposedat the interior center of the hotbox, around which the SOFC stacks andoxidant manifolds are disposed. From placement within the stack hotbox,the reformer also benefits from being in thermal communication with eachstack. In this manner, the steam reformer captures excess stack heat viaradiation and convection. The steam reformer is adapted with an inlet toinput a hydrocarbon fuel, an inlet to input steam, and optionally aninlet to input an oxidant, preferably air or oxygen. In the case of aliquid hydrocarbon fuel, it is desirable to atomize the fuel so as tominimize coke formation within the reformer. In one embodiment, anatomizer is disposed external to the hotbox, and a conduit for feedingan atomized liquid fuel is provided from the externally located atomizerto the fuel inlet of the reformer. In another embodiment, an atomizer islocated within the hotbox, preferably, as a component of the steamreformer such that under operating conditions the liquid fuel isatomized and vaporized utilizing excess heat from the stack.

In one embodiment, the steam reformer operates on a mixturepredominantly comprising the hydrocarbon fuel and steam, with little orno oxidant, in an endothermic steam reforming process. Typically, aburner is integrated with the reformer in a combined steamreformer-burner module, for the purpose of combusting a separate inputof hydrocarbon fuel or combusting a fuel exhaust gas derived from theSOFC stacks, thereby providing heat via combustion to drive theendothermic steam reforming process. In one embodiment, the burnerprovides a primary source of heat to the reformer; whereas heat from thestacks provides a secondary source of heat to the reformer, due topositioning the reformer within the hotbox as well as capturing excessstack heat in the heat extractor to produce or heat steam for thereformer. This embodiment beneficially allows for high stack fuelutilization, which results in a fuel exhaust gas from the stacks that isdepleted in heat content and is not entirely sufficient to drive theendothermic steam reformer by itself. Excess stack heat of the secondaryheat source provides the thermal balance for the reformer in such cases.In another embodiment, the burner is fed with a separate input ofhydrocarbon fuel, when needed, to balance overall system requirements,for example, when the stacks are operating during startup or at highfuel utilization, e.g., greater than about 80 percent fuel utilization.The burner also functions as a start-up burner providing heat forraising the system components to their desired operating temperature(s),particularly, as that pertains to the reforming catalyst and the SOFCstacks. Thus, the burner is utilized during start-up, transients andsteady state operation, as desired.

In another embodiment, the steam reformer operates on supplies of ahydrocarbon fuel, steam, and oxidant in an autothermal reforming (ATR)process. This embodiment functions exothermically with release of heat.By cycling excess stack heat to the ATR process via radiation andconvection as well through steam generation in the heat extractors, theoxidant requirement of the ATR process is reduced. This advantageouslyresults in an increased thermal efficiency of the steam reformer and areduced dilution of the reformate stream exiting the reformer. Asanother option, the steam reformer can be operated under ATR conditionsduring a start-up phase of the SOFC stacks. During the start-up phase,the temperature of all components including the SOFC stacks and thereforming catalyst must be raised to steady-state operatingtemperatures. Thus, the exothermic nature of ATR operation provides heatto cold-start the system components. After steady-state temperatures arereached, the steam reformer is advantageously converted to endothermicsteam reforming status, wherein heat generated by the stacks isrecuperated for use in the heat extractors and the reformer. The burner,which is needed during endothermic steam reforming, can also be utilizedto generate heat during start-up.

The steam reformer employed in this invention comprises any steamreformer as known and described in the art. Generally, the steamreformer comprises a fuel inlet, a steam inlet, and a catalytic reactionzone having disposed therein a substrate onto which a reforming catalystis supported. Non-limiting examples of suitable substrates includepowders, pellets, extrudates, foams, and meshes. In one advantageousembodiment, the substrate is provided as a mesh constructed in the formof a reticulated net or screen comprising a plurality of pores, cells,or channels having an ultra-short-channel-length, as definedhereinafter. FIG. 5 depicts in horizontal transverse view a steamreformer (3) and a burner (21) arranged as two concentric cylinders, theburner (21) occupying the inner cylindrical space and the reformeroccupying an annular outer space. The reformer (3) comprises a meshsubstrate (23) comprising an array of struts (27) and a plurality ofvoid volumes (29), the struts having supported thereon particles ofcatalyst (25). In one embodiment, the mesh is provided in a coiledconfiguration of cylindrical shape having an inner diameter and a largerouter diameter such that reactants flowing there through move along aradial flow path from an inlet along the inner diameter to an outletalong the outer diameter. In another embodiment, the mesh is provided asa stack of planar sheets with an inlet at one end of the stack and anoutlet at an opposite end of the stack. In any configuration the bulkconfiguration of the mesh provides for a plurality of void volumes inrandom order, that is, empty spaces having essentially no regularityalong the flow path from inlet to outlet. The mesh substrate is suitablyconstructed from a metal mesh, a ceramic mesh, or a combination thereofas in a cermet.

In more specific embodiments, the metal mesh substrate is constructedfrom any thermally conductive metal or alloy capable of withstanding thetemperatures and chemical environment to which the substrate is exposed.Suitable non-limiting materials of construction include iron-chromiumalloys, iron-chromium-aluminum alloys, and iron-chromium-nickel alloys.Such metal meshes are available commercially, for example, from AlphaAesar and Petro Wire & Steel. In one exemplary embodiment, the metalmesh comprises a MICROLITH® brand metal mesh (Precision Combustion,Inc., of North Haven, Conn., USA). As described in U.S. Pat. Nos.5,051,241 and 6,156,444, incorporated herein by reference, MICROLITH®brand mesh technology offers a unique design combining anultra-short-channel-length with low thermal mass in one monolith, whichcontrasts with prior art monoliths having substantially longer channellengths as noted hereinafter.

With reference to a ceramic mesh substrate, the term “ceramic” refers toinorganic non-metallic solid materials with a prevalent covalent bond,including but not limited to metallic oxides, such as oxides ofaluminum, silicon, magnesium, zirconium, titanium, niobium, andchromium, as well as zeolites and titanates. Reference is made to U.S.Pat. Nos. 6,328,936 and 7,141,092, detailing layers ofultra-short-channel-length ceramic mesh comprising woven silica, bothpatents incorporated herein by reference. With reference to a cermetsubstrate, the term “cermet” refers to a composite material comprising aceramic in combination with a metal, the composite being typicallyconductive while also exhibiting a high resistance to temperature,corrosion, and abrasion in a manner similar to that of ceramicmaterials.

The mesh substrate is not limited by any method of manufacture; forexample, meshes can be constructed via weaving or welding fibers, or byan expanded metal technique as disclosed in U.S. Pat. No. 6,156,444,incorporated herein by reference, or by 3-D printing, or by a lostpolymer skeleton method.

In a preferred embodiment, the substrate employed in the stream reformerof this hotbox invention comprises the aforementioned MICROLITH® brandmesh of ultra-short-channel-length (Precision Combustion, Inc., NorthHaven, Conn., USA). Generally, the mesh comprises short channel length,low thermal mass monoliths, which contrast with prior art monolithshaving longer channel lengths. For purposes of this invention, the term“ultra-short-channel-length” refers to a channel length in a range fromabout 25 microns (μm) (0.001 inch) to about 500 μm (0.02 inch). Incontrast, the term “long channels” pertaining to prior art monolithsrefers to channel lengths of greater than about 5 mm (0.20 inch) upwardsof 127 mm (5 inches). In this invention the term “channel length” istaken as the distance along one pore or channel as measured from aninlet on one side of the mesh sheet to an outlet on another side of themesh sheet. This measurement is not to be confused with the overalllength of flow path through the entire substrate from an inlet at theinner diameter, for example, of the coiled mesh to an outlet at theouter diameter of the coiled mesh. In another embodiment, the length ofthe pore, cell, or channel is no longer than the diameter of theelements from which the mesh is constructed; thus, the channel lengthmay range from 25 μm (0.001 inch) up to about 100 μm (0.004 inch).Generally, the channel length is no longer than about 350 μm (0.014inch). In view of this ultra-short channel length, the contact time ofreactants with the mesh and catalyst supported thereon advantageouslyranges from about 5 milliseconds (5 msec) to about 350 msec. TheMICROLITH® brand ultra-short-channel-length mesh typically comprisesfrom about 100 to about 1,000 or more flow channels per squarecentimeter.

More specifically, each layer of mesh in this invention typically isconfigured with a plurality of channels or pores having a diameterranging from about 0.25 millimeters (mm) to about 1.0 mm, with a voidspace greater than about 60 percent, preferably up to about 80 percentor more. A ratio of channel length to diameter is generally less thanabout 2:1, preferably less than about 1:1, and more preferably, lessthan about 0.5:1.

The MICROLITH® brand mesh having the ultra-short-channel-lengthfacilitates packing more active surface area into a smaller volume andprovides increased reactive area and lower pressure drop, as comparedwith prior art monolithic substrates. Whereas in prior art honeycombmonoliths having conventional long channels where a fully developedboundary layer is present over a considerable length of the channels; incontrast, the ultra-short-channel-length characteristic of the meshsubstrate of this invention avoids boundary layer buildup. Since heatand mass transfer coefficients depend on boundary layer thickness,avoiding boundary layer buildup enhances transport properties. Employingthe ultra-short-channel-length mesh, such as the MICROLITH® brandthereof, to control and limit the development of a boundary layer of afluid passing there through is described in U.S. Pat. No. 7,504,047,which is a Continuation-In-Part of U.S. Pat. No. 6,746,657 to Castaldi,both patents incorporated herein by reference. The preferred MICROLITH®brand mesh of ultra-short-channel-length also advantageously providesfor a light-weight portable size, a high throughput, thorough mixing ofreactants passing there through, a high one-pass yield ofhydrogen-containing reformate, a low yield of coke and coke precursors,and an acceptably long catalyst lifetime, as compared with alternativesubstrates, such as, ceramic monolith and pelleted substrates.

The substrate disposed within the reforming zone of the steam reformersupports a reforming catalyst, which under operating conditionsfunctions to facilitate steam reforming or autothermal reforming, asdesired. A suitable reforming catalyst comprises one or more metals ofGroup VIII of the Periodic Table of the Elements, including iron,cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium,platinum, and mixtures thereof. The catalyst chosen depends upon theparticular fuel fed to the reformer. Gaseous fuels, such as methane, aresuitably reformed with a nickel catalyst as known in the art. Liquidfuels, such as diesel, are suitably reformed with one or a mixture ofplatinum group metals (PGM, e.g., Ru, Rh, Pd, Os, Ir, Pt. and mixturesthereof). The deposition of catalytic metal(s) onto the metal mesh isimplemented by methods well known in the art. Alternatively, finishedcatalysts comprising catalytic metal(s) supported on the MICROLITH®brand mesh substrate are available from Precision Combustion, Inc.,North Haven, Conn.

In another exemplary embodiment, the mesh is constructed of an analogousstructure of metal, ceramic, or other manufactured or structuredultra-short-channel-length substrate material comprising aninterconnected network of solid struts defining a plurality of pores ofan open-cell configuration. In this embodiment, the pores have any shapeor diameter; but typically, a number of pores that subtend one inchdesignate a “pore size,” which for most purposes ranges from about 5 toabout 80 pores per inch. The relative density of such structures, takenas the density of the structure divided by the density of solid parentmaterial of the struts, typically ranges from about 2 to about 15percent. Manufactured or structured ultra-short-channel-lengthsubstrates are commercially available in a variety of materials capableof withstanding the operating temperatures of the steam reformer andSOFC of this invention.

The reformer further comprises one or more inlets for feeding to thecatalytic reforming zone the hydrocarbon fuel, steam, and optionally ananode tail gas exhausted from the anode side of the SOFC stack(s). Inanother embodiment, an oxidant is additionally fed to the reformer. Inyet another feature, the reformer further comprises a reformer outletfor exiting a reformate stream comprising predominantly hydrogen andcarbon monoxide, the reformate stream passing into the fuel side of eachof the solid oxide fuel cell stacks. The inlets and outlets of thereformer are conventional in design and constructed from any suitablematerial capable of withstanding the temperature and chemicals to whichthe material is exposed.

The steam reformer is typically associated with a burner, whichfunctions to burn a portion of the hydrocarbon fuel, or a separatesource of fuel, such as the anode tail gas exhausted from the SOFCstack(s), or a mixture thereof to provide internal heat to drive thereformer. In one embodiment, the burner is disposed as an integral unitof the steam reformer; for example, the reformer-burner unit comprisestwo concentric tubes in direct conductive thermal communication, such asin a shell-in-tube reactor design. In this embodiment, the inner tubeaccommodates the burner; while the outer annular volume accommodates thesteam reformer, as illustrated in FIG. 5 . In another embodiment, thefunctions are reversed, such that the inner tube accommodates the steamreformer; while the outer tube accommodates the burner. In yet anotherembodiment, the burner is disposed within the hotbox housing in spacedapart relation to the steam reformer. In yet another embodiment, theburner is positioned exterior to the hotbox housing. In each embodiment,the burner is required to be in thermal communication with the steamreformer, so that heat generated in the burner is used to produce steamfor the steam reformer and used to drive the endothermic steam reformingprocess.

The burner is suitably provided by any conventional flame-stabilized orflameless, catalytic or non-catalytic burner design as known in the art.The burner comprises a housing defining a combustion zone or chamber,one or more inlets for feeding to the combustion zone the oxidant andthe hydrocarbon fuel, the anode tail gas, or a mixture thereof; and anoutlet for exiting combustion products. The burner typically may alsoinclude an ignition device. The chamber itself, the inlets and outletare conventionally constructed from any material of suitable durabilityin view of the temperature and chemicals to which the burner is exposed.Suitable non-limiting materials of construction for the burner include,for example, nickel-chromium alloys and nickel-chromium-iron alloys,such as INCONEL®, HASTELLOY®, and HAYNES® brands of alloys. These alloysare appropriately passivated to prevent contamination of components fromoff-gassing. In one embodiment, an interior chamber of the burner isfilled with a combustion catalyst, such as a platinum group metal (PGM)provided in the form of powder, pellets, extrudates, or the like.

Each SOFC fuel cell stack as required of this invention comprises aplurality of individual solid oxide fuel cell repeat units. Each fuelcell repeat unit comprises a sandwich assembly having constituent partsin the following order: a fuel electrode (anode), a solid oxideelectrolyte, and an oxygen electrode (cathode), the fuel and oxygenelectrodes being connected via an external electrical circuit. Eachstack further includes interconnects that connect the fuel electrodesand oxygen electrodes in adjacent cells thereby collecting the currentfrom each cell and delivering the collected currents to the externalcircuit. The stack also includes bipolar plates, which separate theindividual fuel cell repeat units from each other as well as flowmanifolds that deliver and distribute the flows of stack fuel and oxygento their respective electrodes within the stack and remove products fromthe stack.

The solid oxide fuel cell is an apparatus that in forward operationprovides for the electrochemical reaction of a stack fuel, namelyhydrogen or carbon monoxide, with an oxidant, such air or oxygen, toproduce a DC electrical current and a chemical product, namely, water orcarbon dioxide, respectively. In another embodiment, the stack fueladditionally comprises methane. The stack fuel is fed to the fuelelectrode (anode at the anode side of the SOFC stack) where it reactsvia oxidation with oxide ions to produce the oxidized chemical product,i.e., the water and/or carbon dioxide, and a flow of electrons. Theelectrons travel via an embedded current collector and the externalelectrical circuit to the oxygen electrode (cathode at the cathode sideof the SOFC stack), where molecular oxygen is reduced to form oxideions. During transit the electrons are available to do useful work. Theoxide ions produced at the oxygen electrode diffuse through the solidoxide electrolyte to the fuel electrode to complete the chemicalreaction.

The art describes many embodiments of solid oxide fuel cell repeatunits, any of which is suitably employed in this invention. As anon-limiting exemplary embodiment, the solid oxide electrolyte comprisesa ceramic that is a good conductor of oxide ions but a poor ornonconductor of electrons, which ensures that the electrons pass throughthe external circuit. As a non-limiting exemplary embodiment, the solidoxide electrolyte is constructed of a ceramic comprising ayttria-stabilized zirconia (YSZ) sandwiched in between a fuel electrodecomprised of a nickel oxide/YSZ cermet and an oxygen electrode comprisedof a doped lanthanum manganite. This is only one suitable design whichshould not limit the invention in any manner.

It should be appreciated that a fuel exhaust stream (anode tail gas)exits each SOFC stack at the anode side fuel exhaust outlet, the exhaustgas comprising, in addition to water and carbon dioxide, any unreactedhydrogen and carbon monoxide. In one exemplary embodiment applied tothis invention, the fuel exhaust stream or a portion thereof is recycledto the burner and combusted to generate heat, which is recuperated andutilized to drive the endothermic stream reforming process. In anotherexemplary embodiment, the fuel exhaust stream or a portion thereof isrecycled to the steam reformer, thereby minimizing water needs of thereformer, increasing hydrogen content in the reformate stream, andconcomitantly increasing overall system efficiency. In yet anotherexemplary embodiment, the fuel exhaust stream is split into two flows,one of which is fed to the steam reformer and the other of which is fedto the burner.

In this invention, a plurality of SOFC stacks are employed ranging innumber typically from 2 to about 8. It should be appreciated that eachSOFC stack employed in this invention is not limited by any size orshape. Typically, each SOFC stack is provided as a tower, preferably,having a transverse cross-section of a square or rectangular shape. Inthis invention, under operating conditions at high temperatures, eachstack radiates heat from all sides thereby raising the temperature ofthe environs around the stacks within the hotbox. Placement of thereformer at the core of the hotbox and strategic placement of theSOFC's, around the reformer take advantage of the radiant and convectiveheat available within the stack hotbox.

In another embodiment of this invention, the collective design of solidoxide fuel cells and oxidant manifolds is repeated in a plurality ofconcentric rings, all disposed within the hotbox. Thus, one couldenvision FIG. 1 with a second circle of SOFC units (1) and oxidantmanifolds (11) disposed around and outside the circle illustrated inFIG. 1 . This embodiment envisions each fuel exhaust outlet (9) beingfed into an outer ring of stack fuel inlets. Additional stack fuel canbe supplied to each SOFC in each outer ring.

A plurality of oxidant manifolds are disposed around the central streamreformer in spaced apart relationship to each other. Generally, oneoxidant manifold is disposed in between each pair of adjacent SOFCstacks. The manifold itself comprises a housing defining an interiorplenum. As illustrated in FIG. 2 , for example, alternate oxidantmanifolds (11) are equipped with an inlet (13) for feeding an oxidantgas stream, such as air, into the plenum. The remaining alternatingoxidant manifolds (11) are equipped with an oxidant outlet (15) forexiting a gaseous oxidant stream (cathode waste stream) collected fromthe oxidant electrode of each adjacent stack. The interior plenumfluidly communicates with the oxidant electrode (cathode side) of eachadjacent solid oxide fuel cell stack. Each oxidant manifold isconstructed from a material capable of withstanding the oxidantatmosphere and the temperature to which the manifold is exposed.Suitable materials of construction include, without limitation, any ofthose temperature durable metal alloys described hereinbefore.

It should be appreciated that the SOFC stacks and the oxidant manifoldsare geometrically designed to fit into a compact modular unit, forexample, a three-dimensional polyhedron, as exemplified by the octagonalshape shown in FIGS. 1 and 4 . In this design, the SOFC stacks areseamed to the oxidant manifolds only along the exterior edges of eachunit; substantially no contacts and seams are found on the faces of thestacks and the oxidant manifolds where ceramics in the stacks are morelikely to contact metals of the oxidant manifold. Rather, the design ofthe invention minimizes contacts and seals between unlike materials ofconstruction. For example, an SOFC stack embedded in a metal casing canbe easily seamed to a metal oxidant manifold for long-term durability.

At least one heat extractor is positioned adjacent a fuel cell stack,typically, in in between a fuel cell stack and an adjacent oxygenmanifold. Preferably, a plurality of heat extractors is employed; morepreferably, from 2 to 4 heat extractors are employed. Each heatextractor functions to capture waste heat from the nearby SOFC stacksand to transfer the captured heat to water or steam flowing therethrough, resulting in generation of steam or super-heated steam,respectively, for use in the steam reformer. Since water and steam havea high heat capacity, excess heat is efficiently removed in this mannerfrom the stack. The steam so produced is fed into the steam reformer,thereby recuperating excess stack heat to drive the endothermicreforming process while simultaneously reducing the burden of externalheat generated in the burner for such purpose.

Each heat extractor is not limited by size or shape. In one exemplaryembodiment, the heat extractor is provided as a tube having an inlet onone end and an outlet on another end, or a plurality of such tubes woundin serpentine fashion and connected into a collective inlet and outlet.Each heat extractor is constructed from any material capable ofwithstanding the temperature to which the heat extractor and itsassociated inlets and outlets are exposed. The material is required tobe essentially non-reactive towards water and steam and also thermallyconductive, that is, capable of capturing heat from the stack and itsenvirons. Suitable non-limiting materials of construction for the heatextractor include, for example, nickel-chromium alloys andnickel-chromium-iron alloys, such as INCONEL®, HASTELLOY®, and HAYNES®brands of alloys. Under operating conditions water or low temperaturesteam is passed through the flow path of each heat extractor.

Under operating conditions the reformer is typically fed a hydrocarbonfuel comprising, for example, any gaseous hydrocarbon existing in agaseous state at 22° C. and 1 atm pressure (101 kPa), or alternatively,any liquid hydrocarbon that is vaporizable and fed as a vapor to thereformer. Non-limiting examples of suitable gaseous hydrocarbons includemethane, natural gas, ethane, propane, butane, biogas, and mixturesthereof. Non-limiting examples of suitable liquid hydrocarbons that arereadily vaporized include hexane, octane, gasoline, kerosene, anddiesel, biodiesels, jet propulsion fuels and synthetic fuels derived,for example, from Fisher-Tropsch processes, and mixtures thereof.Preferred fuels include methane and natural gas. If employed, theoxidant supplied to the reformer comprises a chemical capable ofpartially oxidizing the hydrocarbon fuel selectively to a gaseousreformate comprising hydrogen and carbon monoxide (syngas). Suitableoxidants include, without limitation, essentially pure molecular oxygen,mixtures of oxygen and nitrogen, such as air, and mixtures of oxygen andone or more inert gases, such as helium and argon.

During steam reforming operation, the quantities of steam and fuelsupplied to the reformer are best described in terms of a steam tocarbon ratio (St/C), which is defined as a ratio of number of moles ofsteam (St) provided per atom of carbon (C) provided in the hydrocarbonfuel. Generally, the St/C ratio is greater than about 2.5:1 and lessthan about 4.5:1. Usually no oxidant is supplied during steam reforming;however, some oxidant may be supplied if desired. The quantities ofhydrocarbon fuel and oxidant supplied during steam reforming are bestdescribed in terms of an O:C ratio, wherein “O” refers to atoms ofoxygen in the oxidant and “C” refers to atoms of carbon in the fuel.Generally, the O:C ratio of oxidant to fuel fed under steam reformingconditions ranges from 0:1 to about 0.5:1.

The steam reformer operates at a temperature greater than about 500° C.and less than about 1,100° C. A suitable weight hourly space velocitymeasured at 21° C. and 1 atm (101 kPa) ranges from about 100 liters ofcombined flow of hydrocarbon fuel, steam, and optional oxidant per hourper gram catalyst (100 L/hr-g-cat) to about 6,000 L/hr-g-cat, whichallows for high throughput. The corresponding gas hourly space velocitymeasured at 21° C. and 1 atm (101 kPa) ranges from about 1,500 liters ofcombined flow of hydrocarbon fuel, steam, and optional oxidant per hourper liter catalyst (1,500 hr⁻¹) to about 100,000 hr⁻¹. A reformingefficiency of greater than about 75 percent and, preferably, greaterthan about 80 percent is achievable relative to the LHV (lower heatingvalue) of the hydrocarbon fuel fed to the reformer. The reformer iscapable of operating for greater than about 1,000 hours withoutindications of coke production and catalyst deactivation.

Under operating conditions, the burner is fed a mixture of an oxidantand a hydrocarbon fuel, or an anode tail gas, or a combination of both,and ignition is provided to initiate combustion. The hydrocarbon fueland oxidant are selected from any of those identified hereinabove foruse with the reformer, with methane or natural gas being the preferredhydrocarbon fuel and air being the preferred oxidant for the burner. Theburner is operated under stoichiometric or preferably “fuel-lean”conditions, namely, at an O/C ratio equal to or exceeding a ratiorequired to convert all carbon and hydrogen atoms in the fuel to carbondioxide and water.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions, or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

The invention claimed is:
 1. A process of producing electricitycomprising: (a) providing an apparatus comprising: (i) a steam reformercomprising a fuel inlet, a steam inlet, a reforming zone disposed influid communication with the fuel and steam inlets, and a reformeroutlet disposed in fluid communication with the reforming zone; (ii) aplurality of solid oxide fuel cell stacks disposed around the steamreformer, further disposed in spaced apart relation to each other and tothe steam reformer; wherein each solid oxide fuel cell stack comprises astack fuel inlet disposed in fluid communication with the reformeroutlet, and further comprises a fuel exhaust outlet further wherein boththe stack fuel inlet and the fuel exhaust outlet are disposed in fluidcommunication with a fuel side of each of the plurality of solid oxidefuel cell stacks; (iii) a plurality of oxidant manifolds disposed aroundthe steam reformer, such that each oxidant manifold is disposed inbetween a pair of adjacent fuel cell stacks; wherein the plurality ofoxidant manifolds alternatingly comprise an oxidant inlet but no oxidantoutlet and wherein a remaining plurality of oxidant manifolds having nooxidant inlet comprise an oxidant outlet and further wherein eachoxidant manifold defines an interior plenum fluidly communicating withan oxidant side of each of the plurality of fuel cell stacks; and (iv)at least one heat extractor disposed in between one of the solid oxidefuel cell stacks and its adjacent oxidant manifold; the at least oneheat extractor having a water-steam inlet and a steam outlet, whereinthe steam outlet is disposed in fluid communication with the steam inletto the steam reformer; (b) contacting a hydrocarbon fuel and steam inthe presence of a reforming catalyst within the reforming zone underconditions sufficient to produce a stack fuel comprising hydrogen andcarbon monoxide; (c) feeding oxygen or air into the plurality of oxidantmanifolds, from which the oxygen or air is passed to the oxidant side ofthe plurality of solid oxide fuel cell stacks where oxide ions areproduced; (d) feeding the stack fuel to the fuel side of the pluralityof solid oxide fuel cell stacks, and contacting said stack fuel with theoxide ions under conditions sufficient to produce water and carbondioxide and a flow of electrons; (e) feeding water into the heatextractor, which extracts heat from the solid oxide fuel cell stacks toproduce steam; and (f) feeding the steam so produced to the fuelreformer.
 2. The process of claim 1 wherein the hydrocarbon fuel is agaseous fuel selected from the group consisting of methane, natural gas,ethane, propane, butane, biogas, and mixtures thereof.
 3. The process ofclaim 1 wherein the reforming zone comprises a porous substrate havingthe reforming catalyst supported thereon.
 4. The process of claim 3wherein the porous substrate comprises an ultra-short-channel lengthmesh, and the reforming catalyst comprises at least one Group VIII metalof the Periodic Table.
 5. The process of claim 1 wherein the steamreformer is integrated with a burner in a shell-in-tube configuration.6. The process of claim 1 wherein the plurality of solid oxide fuel cellstacks number from 2 to
 8. 7. The process of claim 1 wherein theplurality of oxidant manifolds number from 2 to 8.